The usual configuration for boat sized diesels is 30 micron primary and 10 micron secondary. Those sizes keep particles out of the fuel that cause major damage to fuel system components.
But there are advantages to running really clean fuel - longer injector and pump life. I run a 2 micron primary and the filter lasts about 500 hours. But my tanks are clean and I buy clean fuel from dealers that maintain their tanks and filters. Not just any marina.

You need to give us a little more information, different mfgrs. have different recommendations, So what Make, Model, age & type of engine ?
Does engine have filters on it & what are they & do you want to have to change your on engine filters regularly, or would you rather change your before engine RACOR's or what ever you have more often.

Different Diesel Engine Makers have different injector systems, also year of manufacturer means different specs.

KAPND is absolutely correct about the recommended way to do it.

But then the question is - do you have even dual series fuel filters ?

I like to run a smaller filter than 10 microns in my RACORS to save how often I must change the factory fuel filters on the engine.

So I run 2 micron filters & that has worked for me for 4 decades.

30 micron first then a 10 (or 2 micron in my case) next, then your factory on engine filters as back up as no filter stops 100% of everything, 100% of the time.

Thank you one and all. I removed my primary filter out of the hell hole. I mounted a water seperator follow by 2 racors (10 micron) in series. I attached the 3 units to starboard and hung it from a pilot house floor stringer. I now remove the stairs and there it is, life is good

You really should do step filtration, as someone else posted Tony Athen's website which he talks a lot about that, also understand ultra fine can put an extra load on fuel pump. As many have stated you should follow the manufacturers recommendations.
Human nature is if 10 micron is good 2 micron is even better, and that's not always the case.... Good luck!

We have been using a 10 micron primary (Racor 500FG) for the last 22 months (Since we bought Encore), and in the last year, I've had to drain about 1/2" of 'junk' from the filters every few months.

This last week, a trip to Captiva was taken in less than ideal conditions, and we hobby horsed for 3 hours steady. During a routine Engine Room check while underway, I found about 1" of 'junk' at the bottom of one of the Racors (the other was negligible). My manifold allows me to isolate one filter (I use both all the time), which I did, and drained the 'junk'. I filled the filter with fresh diesel, and upon bringing back online, about 1/2 of 'junk' filled the bottom of the clear bowl. During the entire trip home, the engines ran flawlessly at 1700-1800 rpms, but during the 5 hour trip the filter gained almost 1" of (you guessed it) 'junk'. At home, I tested it for water, Negative. I drained the bowl into a clear container and let it settle. Attached is the result.

Alge? Bacteria? Have birds been crapping in my fuel tank?
I replaced the Racor filters with new 10 micron (the old ones had no evidence of green anything in them), but I thought what the heck...Your thoughts?

We have been using a 10 micron primary (Racor 500FG) for the last 22 months (Since we bought Encore), and in the last year, I've had to drain about 1/2" of 'junk' from the filters every few months.

This last week, a trip to Captiva was taken in less than ideal conditions, and we hobby horsed for 3 hours steady. During a routine Engine Room check while underway, I found about 1" of 'junk' at the bottom of one of the Racors (the other was negligible). My manifold allows me to isolate one filter (I use both all the time), which I did, and drained the 'junk'. I filled the filter with fresh diesel, and upon bringing back online, about 1/2 of 'junk' filled the bottom of the clear bowl. During the entire trip home, the engines ran flawlessly at 1700-1800 rpms, but during the 5 hour trip the filter gained almost 1" of (you guessed it) 'junk'. At home, I tested it for water, Negative. I drained the bowl into a clear container and let it settle. Attached is the result.

Alge? Bacteria? Have birds been crapping in my fuel tank?
I replaced the Racor filters with new 10 micron (the old ones had no evidence of green anything in them), but I thought what the heck...Your thoughts?

That's some nasty looking fuel. From the photo it looks like water, which suggests you have a way for water to get into the tank.